1. Field of the Invention
The present invention relates to a method of producing an organopolysiloxane composition, and more particularly to a method of producing a silicone rubber composition containing less amounts of low-molecular siloxanes by polymerizing an organopolysiloxane in the presence of a reinforcing filler.
2. Description of the Prior Art
Since silicone rubbers have such excellent properties as weather resistance, electrical properties, low-compression set, heat resistance, cold resistance, and the like, they are widely used in various fields including, for example, the fields of electronic equipment, automobiles, building, medical treatment, and food. Their typical applications include, for example, rubber contacts used as rubber contact keys of remote controllers, typewriters, word processors, computer terminals, and musical instruments; gaskets for building; various rolls, such as rolls for copying machines, development rolls, transfer rolls, charge rolls, and sheet feeding rolls; rubber vibration insulators of audio equipment or the like; and packings for compact disks used in computers.
The silicone rubbers used in the above applications are provided generally in the form of a composition containing an organopolysiloxane having a high degree of polymerization and a reinforcing filler. This composition is prepared by mixing a raw material, a polymer, with a reinforcing filler and various dispersants by using a mixer, for example, a dough mixer or a two-roll.
Incidentally, as shown, for example, in US Pat. No. 2,634,252, generally, the organopolysiloxane having a high degree of polymerization that is a base component of the above silicone rubber is produced by polymerizing an organopolysiloxane having a low molecular weight in the presence of an equilibration catalyst, such as potassium hydroxide, a potassium silanolate, lithium silanolate, and a tetraalkylammonium hydroxide. Since the thus obtained organopolysiloxane having a high degree of polymerization is highly viscous, there is the problem that, in the production of the above silicone rubber composition, when the organopolysiloxane is mixed with a reinforcing filler, a large amount of energy is needed.
To solve the above problem, for example, Japanese Pre-examination Patent Publication (KOKAI) Nos. 59-176322, 59-176324, 59-176325, and 59-176326 suggest methods wherein an organosiloxane oligomer having a relatively low viscosity is mixed with a filler and is polymerized in the presence of a catalyst, such as sulfuric acid, sulfonic acid, a quaternary ammonium carboxylate, and a carboxylate, thereby simultaneously carrying out polymerization of the organopolysiloxane and preparation of a silicone rubber composition.
According to the above methods, although indeed a desired silicone rubber composition can be obtained using relatively low energy, a step of neutralizing and deactivating the catalyst used is required and, further because the organopolysiloxane is polymerized by equilibration, low-molecular siloxanes are produced concomitantly, and a step of removing the low-molecular siloxanes is required, which is a defect.
Further, at present, in various fields, it is strongly demanded that low-molecular siloxanes contained in silicone rubbers are reduced. For example, in the case of contact rubbers used for contact keys, there is the problem that low-molecular siloxanes cause a contact failure, and in the case of roll materials for copying machines, there is the problem that an offset phenomenon occurs, that is, a toner is transferred onto the roll through low-molecular siloxanes. In the case of building gaskets or the like, it is known that the substance that causes their surroundings to be soiled is low-molecular siloxanes remaining in the organopolysiloxane that is the raw material.
The production methods suggested in the above prior art require an additional step to reduce low-molecular siloxanes and is not satisfactory in the attainable level to which low-molecular siloxanes are reduced.